The metabolism of carbazeran has been investigated in vitro using liver cytosol from dog, baboon and man. Carbazeran was not metabolized in cytosol prepared from dog liver but was rapidly metabolized to a single product in baboon- and human-liver cytosol. The product was identified as 4-hydroxy carbazeran. The enzyme responsible for the 4-hydroxylation of carbazeran in vitro was shown by the use of inhibitors to be liver aldehyde oxidase. Species differences in the metabolism of carbazeran in vitro correlate well with studies in vivo; these showed that following an oral dose to man and baboon, the compound was almost completely cleared via pre-systemic 4-hydroxylation, whereas in the dog, this metabolic route appeared unimportant.